Digital pulse-width-modulation control of a radio frequency power supply for pulsed laser

ABSTRACT

A pulse width modulation method for controlling the output power of a pulsed gas discharge laser powered by a pulsed RF power supply comprises delivering a train of digital pulses to the RF power supply. Each pulse in the train has an incrementally variable duration. The power supply is arranged to deliver a train of RF pulses corresponding in number and duration to the train of digital pulses received. The average power in the RF-pulse train can be varied by incrementally varying the duration of one or more of the digital pulses in the digital pulse train. The train of RF pulses is used to power a gas discharge laser. The gas discharge laser outputs a pulse train corresponding to the RF pulse train.

PRIORITY CLAIM

This application claims priority of U.S. Provisional Application No.61/251,162, filed Oct. 13, 2009, assigned to the assignee of the presentinvention and the complete disclosure of which is hereby incorporated byreference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to carbon dioxide (CO₂) gasdischarge lasers powered by a radio frequency (RF) power supply. Theinvention relates in particular to methods of pulse width modulation(PWM) for selectively varying and controlling the average power outputof the RF power supply.

DISCUSSION OF BACKGROUND ART

A CO₂ gas discharge laser is typically powered by a high-voltage RFpower supply (RFPS). The power supply applies RF voltage to electrodesof the gas laser, which excite a discharge in a lasing gas mixtureincluding CO₂ and inert gases. The discharge takes place within a laserresonator. The discharge energizes the lasing gas such that theenergized gas provides optical gain causing laser radiation to circulatein the laser resonator. A fixed, predetermined portion of thecirculating radiation is coupled out of the laser resonator as outputradiation. The laser is typically operated in a pulsed manner anddelivers pulses at a predetermined peak power, for a given pulseduration, and at a predetermined pulse repetition frequency (PRF).Typically the PRF is between about 1 kilohertz (kHz) and 200 kHz. Theaverage power in a laser pulse is related to the average power deliveredby the RF power supply during the duration of the pulse. The RF powersupply typically operates at a predetermined fixed (RF) frequencybetween about 10 megahertz (MHz) and 150 MHz with 100 MHz being typical,i.e., much higher than the highest contemplated PRF of the train ofpulses.

The power in the laser output pulses is controlled by modulating thewidth of the individual RF pulse from the RF power supply. This powercontrol method is called pulse width modulation (PWM). The RF powersupply is periodically turned (fully) on and (fully) off, therebygenerating a train of RF pulses which are provided to the laserdischarge. The RF pulses in the train have the same on time, and thesame off time between pulses. The pulse train is characterized by a dutycycle which is equal to the pulse duration of one pulse within the pulsetrain divided by the repetition period of the pulse train. RF powerdelivered to the laser is controlled by varying the duty cycle, which iseffected by varying the duration (modulating the temporal width) of theRF pulses during the repetition period. Whatever the duty cycle, thewidth of all RF pulses in a train thereof is the same.

The duration of the pulses in a digital pulse width modulator (DPWM) isdigitally controlled, so a pulse in a train can only be lengthened orshortened by fixed increments, the length of an increment beingdetermined by the frequency of a system clock delivering clock pulses.Similarly the number of RF pulses in a train is fixed (again digitally)at some value required to provide that the train average power can beconsidered as equivalent to a steady state value that the train isattempting to simulate. Accordingly the resolution, i.e., the accuracyto which the average RF power can be controlled, and the correspondingpower of a laser pulse, is determined by the clock-pulse period relativeto the repetition period of the RF pulse train.

By way of example if a DPWM has a clock frequency f=10 MHz, each clockcycle period is 1/f=0.1 microsecond. If the laser PRF=1 kHz(corresponding to the frequency of delivery of RF pulse trains) acomplete pulse width modulation period would contain 10 MHz/1 kHz=10,000clock cycles and the resolution would be 10,000, i.e., 0.01%. If thelaser PRF is increased to 100 kHz with the same clock frequency theresolution falls to 10 MHz/100 kHz=100, i.e., 1.0%. In order to obtainthe resolution possible in the 1 kHz-PRF case at 100 KHz, the clockfrequency would have to be increased to 1 gigahertz (GHz). This higherfrequency is not practical in a commercial laser as it requires the usesof correspondingly faster circuit components and wider counters, all ofwhich increases the cost of a laser.

In laser processing application for which CO₂ lasers are used, forexample in semiconductor device processing applications, there is anincreasing trend towards using higher pulse repetition frequencies, forexample, up to 200 kHz or greater. Power control accuracy significantlybetter than 1% is generally desired. There is a need for a PWM methodthat would allow this control accuracy with reasonable clockfrequencies, for example between about 1 MHz and 10 MHz.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus forcontrolling, by pulse width modulation, the output power of a pulsed gasdischarge laser powered by a pulsed RF power supply. In one aspect ofthe present invention, the pulse width modulation method comprisesdelivering a train of digital pulses to the RF power supply. The trainhas a predetermined number of pulses therein, and each pulse in thetrain has an incrementally variable duration. The power supply isarranged to deliver a train of RF pulses corresponding in number andduration to the train of digital pulses received, each train of RFpulses having an average power dependent on the duration of the RF-pulsetrain and the aggregate duration of pulses in the RF-pulse train. Theaverage power in the RF-pulse train can be varied by incrementallyvarying the duration of one or more, but less than all, of the digitalpulses in the train thereof.

In a preferred embodiment of the inventive pulse width modulationmethod, the duration of the digital pulses in the train thereof iscontrolled by pulses delivered by a digital clock. The incrementalvariation of the duration of the one or more digital pulses is one ormore of the pulse repetition periods of the digital clock. The inventivepulse width modulation method is referred to herein as a dual modulusdigital pulse width modulation (DMDPWM) method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, and 1D are graphs of voltage as a function of timeschematically illustrating principles of the (DMDPWM) method of thepresent invention and depicting digital pulse parameters as a functionof clock cycle periods in a simplified example of the method.

FIG. 2 is a high level circuit block diagram schematically illustratingone preferred embodiment of digital pulse width modulator circuitry forimplementing the method of the present invention including a signalprocessor for translating user requests into three digital words, periodand pulse width counting circuitry responsive to two of the digitalwords and N-modulo counter circuitry responsive to the other digitalword.

FIG. 3 is a circuit diagram schematically illustrating one preferredconfiguration of the N-modulo counter circuitry of FIG. 2

FIG. 4 is a logic circuit diagram schematically illustrating a preferredconfiguration of the period and pulse-width circuitry of FIG. 2, anddetails of the interaction of that circuitry with the N-modulo countercircuitry of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A-D schematically illustrate digital pulse parameters as afunction of clock cycle periods in a simplified example of dual modulusdigital pulse width modulation (DMDPWM) method of the present invention.The parameters are a frame (pulse train) width (integer value) F, here,comprising 4 pulses (F=4); and a period P (integer value), here,comprising 5 clock cycle periods t (P=5 t). Note that pulse frames wouldnormally be delivered repetitively in practical use of a laser but maybe given different parameters from one repetition to the next, asneeded.

The clock cycle period t is equal to 1/f, where f is the clockfrequency. In FIG. 1A, all of the pulses have a basic width (duration) Wequal to 2 t. The voltage amplitude of the pulses is represented by V.As summarized above, the power delivered in a pulse frame can be givenvarious average values by incrementally increasing (stretching) theduration of one or more pulses in the frame beyond the basic duration.The number of stretched pulses in a frame is designated by aninteger-parameter N. In the example of FIG. 1A there is no stretching(N=0) of any of the pulses within the frame F=4.

In the example of FIG. 1B, one pulse (the fourth in the frame of fourpulses) is stretched (N=1) within the frame of 4 pulses. The amount ofstretching is one clock-cycle period t.

In the example of FIG. 1C, two pulses (the second and fourth in theframe of four pulses) are stretched (N=1) within the frame of 4 pulses.The amount of stretching for each of the stretched pulses is oneclock-cycle period t.

In the example of FIG. 1D each of the four pulses in the frame isstretched by one clock-cycle period. In this case, as all of the 4pulses within the frame are stretched the basic pulse width of eachpulse increases to W=3 clock-cycle periods. Accordingly, N=0 again as inthe example of FIG. 1A.

It should be noted here that the pulse trains of FIGS. 1A-D are trainsof digital pulses that are delivered to the RFPS by inventive DMDPWMcircuitry described in detail further hereinbelow. In a RF-driven gasdischarge laser, the pulse trains would command an RFPS to delivercorresponding trains of laser output pulses of RF energy, which wouldproduce corresponding pulses of laser energy. The envelope of the laseroutput pulses would be similar to the envelope of the digital pulsesexcept for slower rise and fall times. The term envelope is used here inrecognition that the RF pulses would be voltage varying at RF frequencyunder the envelope.

The average power of a frame of pulses can be represented by a dutycycle D, which is the sum of the duration of all pulses within the framedivided by the duration of the frame. The average value of the pulsetrain determines the average power of laser output. As noted above, inprior-art DPWM methods the width of all pulses in a train is incrementedto increase the duty cycle. Accordingly, the resolution is limited bythe number of clock cycles in a pulse repetition period of the laser. Inthis inventive method, wherein the width of individual pulses in a framecan be incremented, the resolution is effectively increased by 1/F whereF is the number of pulses in a frame than can be stretched.

Stretched pulses can be evenly distributed throughout a frame. Thisresults in a smoother output waveform than occurs in the case where allthe stretched pulses are bunched together. This smoothing of the outputwaveform is important for minimizing the peak-to-peak amplitude rippleof the output of the RFPS driving the laser. This smoothing of the RFPSoutput, translates to a smoother power output from the laser. Stretchingthe duration of pulses from a basic value by only one clock cycle, whichcan be a very small time increment, and which can be important inminimizing this ripple.

By way of example, in a case where N=0 and F is some number of pulsesthat can be stretched, then the average value of the wave-form with peakamplitude V, pulse width W and a period P emitted out of the DMDPWM isgiven by an expression:VWF/PF=VD  (1)where D=W/P and is the duty cycle of the wave form. If the pulse width,being a number W of clock cycle periods t, of a number N of F pulses ina frame of pulses, is increased by one clock cycle period “t”, the valueof the output wave form from the DMDPWM is given by an expression:VW(1+F−N)/PF+VNT/PF  (2)For N=1, expression (2) reduces toV(D+d)  (3)where d=t/FP is the increase in the duty cycle of the pulse train andD=W/P, the duty cycle.

For this example where one of the pulses within a frame is increased byone clock cycle period t, the effective duty cycle control resolutionhas been improved by 1/F. If F consists of 8 bits, giving a frame of 256pulses, the duty cycle control resolution improvement is 1/256 or0.0039. This level of precision is critical in obtaining accurate closedloop control of an RFPS having a high PRF that provides ON/OFF power toa closed-loop controlled gas laser. The advantage of the DMDPWM approachis that high resolution can be obtained with relatively low clockfrequencies while maintaining the high resolution as the PRF isincreased without having to change the clock frequency.

Further by way of example, if it is desired to improve the resolution ofan 8-bit basic DPWM providing a train of 256 pulses by implementing thebasic DPWM as a DMDPWM device in accordance with the present invention,the resolution is improved by stretching some of the pulses by one clockcycle t. For stretching the pulses, pulse width information is providedto the DMDPWM by a signal processor in response to a users input. Thisinformation is defined by a digital word “W”, which specifies theduration, in clock cycles, of an un-stretched pulse, and a digital word“N” which specifies the number of pulses in the pulse train to bestretched. If N=0, no pulses are stretched; If N=1, one pulse out ofevery 256 pulses is stretched; and so on. If N=255, every pulse but oneis stretched. If N is incremented past 255, N rolls back to zero andgenerates a “carry” which is used to increment W. The result is that 256pulses out of 256 can be stretched. Effectively, W and N can beconcatenated into a single digital word WN where each move of one bitposition to the left represents a 2× increase in duty cycle.

FIG. 2 is a high level circuit block diagram schematically illustratinga preferred embodiment 10 of a dual modulus digital pulse widthmodulator (DMDPWM) in accordance with the present invention. The DMDPWMincludes a signal processor (microprocessor) 12 including a system clock14. The DMDPWM also includes pulse width modulating circuitry 16including a period and pulse width (Period/PW) counter 18 and anN-modulo counter 20. Clock 14 delivers clock pulses to the Period/PWcounter. A user inputs to the signal processor a desired pulserepetition period P₁, a desired basic (minimum) pulse width W₁, and adesired resolution in the form of a number of pulses N1 to be stretched.The signal processor translates these inputs into digital words P (22),W (24) and N (26). The digital P and W words are provided to counter 18and the digital N word is provided to counter 20.

The circuitry functions as follows.

Every time counter 18 counts clock pulses up to a period P, the counterresets to zero and the signal out of the DPWM i.e., out of counter 18,to the RF power supply goes to a high value. Every time the counter 18counts up to W, i.e., the basic pulse width, the signal out of the DPWM,to the RFPS goes to a low value. Part of the DPWM out signal is directedto the N-modulo 256 counter 20 to serve as a clock for the N-modulo 256counter. Every time the PWM out signal goes high, counter 20 advances byone count. Counter 20 produces a high output signal N times out of 256pulses of the DMDPWM. Whenever the output signal of counter 20 is high,the pulse width is W+1 instead of W. Counter 20 provides this carry outinformation to counter 18 as indicated in FIG. 2.

To minimize the “ripple” in the RFPS output (and correspondingly laseroutput) caused by the insertion of W+1 duration (stretched) pulses amongW duration (un-stretched) pulses in the output pulse train, it isdesirable that the stretched pulses be distributed relatively uniformlythroughout the sequence of 256 DPWM output pulses, rather than “bunchedup” within the sequence. FIG. 3 schematically illustrates one example ofan arrangement of N-modulo 256 counter 20 which accomplishes this task.

Here, counter 20 includes an 8-bit adder 28 and an 8-bit D flip-flop 30.It should be noted here that adder 28 and flip-flop 30 should handle thesame number of bits, whatever that number of bits may be. Here, 8 bitsare used to improve the basic PWM resolution by 256, i.e., 2⁸.

Every time flip-flop 30 is clocked by the output of the counter 18 ofFIG. 2, the contents of flip-flop 30 are incremented by N. The output offlip-flop 30 can be thought of as the “present state” of circuit 20, andthe input of the flip-flop can be thought of as the “next state” ofcircuit 20. Note that the present state is applied to the input of 8-bitadder 28 together with the digital word N. Adder 28 sums these twoquantities to form the next state. In other words, the next state=thepresent state+N.

When the clock-input of 8-bit D flip-flop 30 goes from low to high, thedata at the input (D) of the flip-flop is transferred to the output (Q)of the flip-flop. The result is that circuit 20 counts by N. Thecarry-out output only has a high signal following those clock cycles inwhich the results of the addition exceed 255. By way of example, if thecounter state is 0 and N=1, the counter will count by ones (1, 2, 3,etc.), and clearly it will overflow every 256 clock cycles. If N=2 thecounter will count by twos (2, 4, 6, etc.), and will overflow after 128(that is 256/2) cycles. The behavior of the counter is more complex whenN is not an integer factor of 256 (for example, N=3), but over a longterm, N output pulses will be produced for every 256 clock cycles, andthey will be distributed over the sequence of 256 clock cycles, ratherthan bunched together within the sequence.

The present invention is described above in a context of extending theresolution of a basic (prior-art) DPWM by 8 bits. The choice of 8-bits,here is arbitrary, but practical. The resolution increase, however, canbe chosen to increase by a greater or lesser amount. By way of example,if it were desired to improve the resolution by 10 bits, an N-modulo1024 counter could be used, and the “stretched” pulses would bedistributed over frames of 1024 output pulses. The resolution, in theoryat least could easily be extended to an even higher number of bits. Atsome level, however, there will be a diminishing of returns becauseperiodic ripple components at some fraction of the laser outputfrequency will be generated.

Circuitry 16, functionally described above with reference to FIG. 2 andFIG. 3, can be implemented in a single commercially available complexprogrammable logic device (CPLD). The inventive circuitry wasexperimentally tested in a model EPM240T CPLD available from the ALTERACorporation of Santa Clara, Calif. Those skilled in the electronic artswill recognize, from the description of the present invention presentedherein, that logic circuitry in accordance with the present inventioncould be implemented in other programmable logic devices, or even in aplurality of individual logic devices, without departing from the spiritand scope of the present invention.

Circuitry 16 of FIG. 4 operates as follows. A counter 32 in circuitry 18counts up by one count with every transition of the clock signal 14 fromthe signal processor (see FIG. 2). The output of counter 32 is fed totwo digital comparators 34 and 36. Whenever the data at the A and Binputs of any one of the comparators are equal, the output of thatcomparator goes to logic 1. If A and B are not equal in any of thecomparators, the output of that comparator will go to logic zero.

A PWM output pulse cycle (train of pulses) begins when output signal ofcounter 32 equals the value P input from the signal processor. Theoutput of comparator 36 goes to logic 1, causing counter 32 to reset tozero count, and setting a Set-Reset (SR) flip-flop 38 to logic 1. Thismarks the beginning of a PWM output pulse out of the SR flip-flop 38.

Counter 32 resumes counting from zero, and when the counter outputequals the value of the digital word W input, the output of digitalcomparator 34 goes to logic 1. It assumed, here, that AND gate 40following comparator 34 is enabled. This being the case the logic 1 fromcomparator 34 propagates through the AND gate and through an OR gate 42to the reset (R) input of SR flip-flop 38 resetting the output of theflip-flop to logic 0. This marks the end of the PWM output pulse. ThePWM output will then remain at logic 0 until counter 32 has againcounted up to the value of the digital word P. When this happens, thePWM Output out of SR flip-flop 38 will again be set to logic 1, and thenext PWM output pulse cycle will begin.

If AND gate 40 had not been enabled when the output of the comparator 34went to logic 1, the output of the comparator would not have propagatedimmediately to the reset input of SR flip-flop 38. Instead, the Q outputof a D flip-flop 44 is acting as a one clock-cycle delay element. Inthis case, the reset input of the SR flip-flop receives its signal toterminate a pulse one clock cycle later than it would have if AND gate42 had been enabled. The PWM output pulse accordingly is “stretched” byone clock cycle.

Circuitry 20, comprising adder 28 and D flip-flop 30 (cooperative withan inverter 46 in circuitry 18) “decides” if AND gate 40 should beenabled or not, i.e., if the PWM output pulse should be “normal” or“stretched”. The operation of circuitry 20 for making the “decision” isdescribed above with reference to FIG. 3.

In summary, the present invention is described above in terms of apreferred and other embodiments. The invention is not limited, however,by the embodiments described and depicted. Rather, the invention islimited only by the claims appended hereto.

1. A pulse width modulation method, for controlling the output of an RFpower supply for a gas discharge laser, comprising: delivering a trainof digital pulses having a predetermined number of pulses therein to theRF power supply, each pulse in the train having an incrementallyvariable duration; arranging the power supply to deliver a train of RFpulses corresponding in number and duration to the train of digitalpulses received, the train of RF pulses having an average powerdependent on the duration of the RF pulse train and an aggregateduration of pulses in the train; and incrementally varying the durationof one or more, but less than all, of the digital pulses in the trainthereof to selectively vary the average power in the corresponding RFpulse train wherein the duration of the digital pulses is controlled bypulses delivered by a digital clock having a clock-cycle period, andwherein the variation of the duration of individual pulses is a multipleof one clock cycle period.
 2. The method of claim 1, wherein theduration of a plurality of pulses in the train thereof is incrementallyvaried and the incrementally varied pulses are distributed about evenlyin the train.
 3. The method of claim 1, wherein pulses in the digitaltrain of pulses have a basic duration of one or more clock-cycle periodsand the incremental variation of the duration of the one or more of thedigital pulses is effected by increasing the duration of the one or morepulses by one or more clock-cycle periods.
 4. The method of claim 3,wherein the duration of a plurality of pulses in the train thereof isincrementally increased, and wherein the incrementally increasedduration pulses are distributed about evenly in the train.
 5. The methodof claim 4, wherein there are N pulses in the train thereof, theduration of N/M of the pulses where M is an integer divisible into N toprovide an integer result and the duration of every M^(th) pulse in thetrain thereof is increased by one clock-cycle period.
 6. The method ofclaim 5, wherein there are 256 pulses in the train thereof.
 7. Gasdischarge laser apparatus comprising, comprising: a radio frequencypower supply (RFPS) for exciting a gas discharge in the laser apparatus;a pulse width modulator arranged to deliver repeated trains of digitalpulses each of the trains having the same duration and the samepredetermined number of pulses therein to the RFPS, each pulse in eachof the trains having an incrementally variable duration, the RFPS beingarranged to deliver a train of RF pulses corresponding in number andduration to the train of digital pulses received, each train of RFpulses having an average power dependent on the duration of the RF pulsetrain and an aggregate duration of pulses in the train; the power supplyto; and wherein the average power in any one of the pulse trainsdelivered by the RFPS relative to another can be selectively varied byincrementally varying the of duration of one or more, but less than all,of the digital pulses in the corresponding train thereof wherein theduration of the digital pulses is controlled by pulses delivered by adigital clock having a clock-cycle period, and wherein the variation ofthe duration of individual pulses is a multiple of one clock cycleperiod.
 8. The apparatus of claim 7, wherein if the duration of aplurality of pulses in the train thereof is incrementally varied, theincrementally varied pulses are distributed about evenly in the train.9. The apparatus of claim 7, wherein pulses in the digital train ofpulses have a basic duration of one or more clock-cycle periods and theincremental variation of the duration of the one or more of the digitalpulses is increasing the duration of the one or more pulses by one ormore clock-cycle periods.
 10. The apparatus of claim 9, wherein theduration of a plurality of pulses in the train thereof is incrementallyincreased, and wherein the incrementally-increased-duration pulses aredistributed about evenly in the train.
 11. The apparatus of claim 10,wherein there are N pulses in the train thereof, the duration of N/M ofthe pulses where M is an integer divisible into N to provide an integerresult and the duration of every M^(th) pulse in the train thereof isincreased by one clock-cycle period.
 12. The apparatus of claim 11,wherein there are 256 pulses in the train thereof.
 13. A pulse widthmodulation method, for controlling the output of an RF power supply fora gas discharge laser, comprising: delivering a repeated trains ofdigital pulses each thereof having the same predetermined number ofpulses therein to the RF power supply, each train of pulses having thesame duration, and each pulse in each of the trains having anincrementally variable duration; arranging the power supply to deliver arepeated trains of RF pulses each train of RF pulses corresponding innumber and duration to the trains of digital pulses received, each trainof RF pulses having an average power dependent on the duration of the RFpulse train and an aggregate duration of pulses in the train; andselectively varying the average power in one of the RF pulse trainsrelative to another of RF the pulse trains by incrementally varying theof duration of one or more, but less than all, of the digital pulses inthe corresponding train thereof wherein the duration of the digitalpulses is controlled by pulses delivered by a digital clock having aclock-cycle period, and wherein the variation of the duration ofindividual pulses is a multiple of one clock cycle period.
 14. A circuitfor supplying RF pulses to a gas laser comprising: a pulse generationcircuit for generating electrical pulses; and an RF power supply forreceiving the electrical pulses and generating pulses of RF energy to bedelivered to the laser with the number and length of the pulses of RFenergy corresponding to the number and length of the electrical pulsesand with the pulse generation circuit being operable to individually andindependently adjust the length of the electrical pulses in order toadjust the average RF power delivered to laser wherein the duration ofthe electrical pulses is controlled by pulses delivered by a digitalclock having a clock-cycle period, and wherein the variation of thelength of individual pulses is a multiple of one clock cycle period.